High-resolution optical spectrometers are used to observe spectral features of an unknown signal. Some high-resolution optical spectrometers implement a heterodyne architecture based upon principles of coherent optical spectral analysis to achieve very fine measurement resolution (also known as coherent optical spectral analyzers). In accordance with this heterodyne architecture, current coherent optical spectral analyzers utilize a 2×2 optical coupler to combine the unknown signal with a local oscillator signal. The local oscillator signal is set to oscillate at a known frequency or is swept across a range of frequencies. The two outputs of the coupler are detected through a nonlinear detector, such as a photodiode, and the resulting electrical signals are subtracted from one another to isolate the desired heterodyne signal. From this, the spectral features of the unknown signal can be obtained.
To perform a measurement utilizing a coherent optical spectral analyzer, the local oscillator signal is swept across different wavelengths, while the heterodyne signal due to mixing with the unknown signal is acquired. Unfortunately, the current receiver architecture, which is based on a 2×2 optical coupler, is unable to measure the precise phase of the heterodyne signal.
The problem with measuring the phase of the heterodyne signal stems from the basic phase ambiguity of a sinusoidal function. Typically, the heterodyne signal as described above will have the general form:H(t)=V(t)cos(Δωt+Δφ(t))  (1)as shown in Equation 1, where Δω represents a frequency difference between the local oscillator and unknown signal, and Δφ(t) represents the relative phase of the heterodyne beat signal. A single measurement of H(t) is unable to resolve Δφ(t), the desired heterodyne phase, because there are two unknowns (V(t) and Δφ(t)).
Accordingly, optical spectrum analyzers, according to the conventional art, try to measure V(t) while ignoring Δφ(t). However, amplitude uncertainty is introduced into the spectral measurement of V(t), because the phase of the heterodyne signal varies throughout the scan, as well as from scan to scan. Furthermore, the inability to observe the phase of the heterodyne signal also results in the receiver being equally sensitive to both positive and negative heterodyne beat frequencies. Therefore, attempts to reduce phase uncertainty by using a bandpass receiver will result in the formation of spectral images that limit the ultimate resolution of the device.